The use of multiple antennas in wireless communications systems is well known to provide significant performance advantages. In particular, links can be made much more robust to multipath fading. This is particularly the case in the emerging 5.9 GHz Car to Car and Car to Infrastructure communications. These are jointly referred to as Car-2-X.
In automotive applications it is desirable to reduce the cost of systems to be deployed to vehicles. Multiple antenna arrangements require extra hardware over single antenna systems. Vehicle manufacturers may want to space antenna elements apart by a meter or more. For example a pair of antennas may be deployed at the rear and at the front of the vehicle roof, or on either side of the vehicle in the mirrors.
Optimal diversity performance is obtained when signals from each antenna are processed using independent RF frontend and digital baseband processing up until the point of demodulation, as illustrated in FIG. 1. Each antenna (101A and 101B) would have its own RF module (102A and 102B) for frequency down-conversion, front end module (FEM) (104A and 104B) for packet acquisition, Automatic Gain Control, synchronisation and channel estimation. The information output from the FEM (104A and 104B) may be partially demodulated by a partial demodulator (106A and 106B). The demodulator 111 combines the signals from all antennas (e.g. 1, 2 or more) using the channel estimate and the observed signal. The channel estimate includes a complex signal level and noise power. The demodulator 111 may then provide its output to a forward error correction (FEC) decoder 112 for providing a decoded output.
In OFDM systems (such as IEEE 802.11p and ETSI TC-ITS G5) this processing is done in the frequency domain (after the application of an FFT) with each OFDM symbol being processed by an FFT resulting in N data bearing symbols per FFT application. In IEEE 802.11p there are 48 data bearing symbols output for each FFT. Phase Shift Keying (PSK) and Quadature Amplitude Modulation (QAM) are typical modulation formats applied to the signals. At the receiver the demodulator's task is to map received symbols corresponding to transmitted symbols to set of bit log-likelihood-ratios (LLR) for each bit used to construct the symbol. For example in 64-QAM there are six bits used to identify a constellation point. Sequences of these LLRs are processed by Forward Error Correction (FEC) decoders to create estimates of the transmitted information bit sequence.
The OFDM multiple antenna receiver is illustrated in FIG. 2. Transmitter details are not shown, and neither is Block Deinterleaver and Interleaver detail shown in the FEC Decoder 212. The FEC decoder 212 will update one OFDM symbol at a time as the bits for each OFDM symbol are normally interleaved in a Block interleaver at the transmitter. This interleaving is undone at the receiver by deinterleaving the LLRs.
The detail of the prior art demodulation for OFDM modulated signals such as IEEE 802.11p is shown in FIG. 2, which shows an example with two antennas.
Each antenna (201A and 201B) configured to receive a signal transmitted across a channel may provide the received signal to a respective RF module (202A and 202B) for frequency down-conversion. The frequency down-converted signal may be provided to a respective receiver digital front end module (FEM) (204A and 204B). Each FEM (204A and 204B) associated with a respective antenna performs functions such as analogue to digital conversion (ADC), filtering, Automatic Gain control and synchronisation (time and frequency). The resulting time domain signal is fed one OFDM symbol at a time to the respective FFT module (206A and 206B).
The FFT module (206A and 206B) transforms the time domain signal to the frequency domain resulting in a block of modulated symbols (often referred to as subcarriers). These symbols are each impaired in transmission by fading and noise. The nature of this fading and noise changes during the packet in Car-2-X use.
Channel estimation of the channel impairing each of these subcarriers is performed by channel estimate modules 208A and 208B. The outputs of the FFTs 206A and 206B and the outputs of the channel estimators 208A and 208B are fed to LLR block 210. Complex channel magnitudes and noise power are estimated for each antenna independently. These estimates are used in the LLR block 210 to calculate the bit log-likelihood ratios (LLRs) for each bit constituting each sub-carrier symbol. The resulting sequence of bit LLRs is fed to the FEC Decoder module 212. The FEC Decoder module 212 may also include an internal block interleaver if the transmitter interleaved encoder outputs. This interleaver would be applied prior to processing the Forward Error Correction.
Diversity is optimally exploited when the baseband signals from two or more antennas are employed in the demodulation process. This joint demodulation is then passed to a single Viterbi decoder and the information bearing bits released to upper layers of the communication protocol (for subsequent processing such as descrambling, CRC checking etc).
Problems occur when two or more antennas are required and they are to be separated by advantageously large distances (e.g. a meter or more). Larger distances offer better spatial diversity and more flexible installation for the Vehicle Manufacturers. However they present problems for the receiver system designer. The two or more antenna elements are to be separated but their signals must merge for Joint Demodulation and processing by a single FEC Decoder. This implies that there is a long connection in at least one of the paths:                1/Antenna to RF: Long RF cables between Antenna and RF module (Coaxial cables are expensive and are also bad for transmit power);        2/Baseband digital samples: Long very high speed digital link (High speed digital links require expensive cables to avoid interference).        
Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in any jurisdiction or that this prior art could reasonably be expected to be understood, regarded as relevant and/or combined with other pieces of prior art by a person skilled in the art.